My questions are: can any waywards explain how he could not think of the risk to his children's health by having sex with a very promiscuous woman? How can a parent who loves their children do this?
There is a thread in the ICR (I Can Relate) forum for BSs to ask questions of WSs. You may find help there. I'm a BS, too. There's also another thread in the ICR forum for those who found out years later.
My XWH is diagnosed as having narcissistic personality disorder (NPD), so may answer won't necessarily jive with your situation. Basically, XWH felt entitled to do what he wanted, and that included having his A. He treats people as objects to be manipulated. I'm not sure he ever loved me or the children, and he didn't think he'd get caught so there would be no consequences.
How can I trust him when he swore on my kids lives he hadn't cheated several times over the years?
He will have to rebuild the trust, and then you'll probably never trust him 100% again. It takes consistent actions over time to rebuild the trust. The layers of lies over such a long time is going to probably add to the time & action to rebuild trust.
he risked my newborn baby catching a serious std has enraged me!
Not only that, dear lady, he put YOUR LIFE at risk. HPV can turn to cancer and kill you. We have had members who found out about the cheating because of their cancer diagnosis. Some have passed and another member says that her uterus is falling out in chunks.
Can he change his conflict avoidant, people pleasing, selfish, lying ways?
Maybe, maybe not. It takes a lot of work and he has to want to change. Unfortunately, only time will tell. Hopefully he's in IC to work on changing into a safe partner.
I know he is remorseful (now he has finally faced his shame) but is he capable or is he a pathological liar?
That's kinda the million dollar (or million pound) question, isn't it? It's really too early to tell. It's that time & actions thing again.
Even though it has been a long time since he said the A was over, you found out so recently that it's still early days for you. They say it takes 2-5 years for you to heal (can take longer, especially if you remain with your WH), and the clock resets to 0 when you find out new stuff.
Be kind to yourself and take care of you and your children.